1. Field of the Invention
The present invention relates to a wire electric discharge machining method and a wire electric discharge machine for high-precision machining of a cutting edge of a rotary-type cutting tool that is mounted with, as a cutting edge, a polycrystalline diamond (PCD) material or polycrystalline cubic boron nitride (PCBN) material, which are the hardest cutting tool materials.
2. Description of the Related Art
Herein, a polycrystalline diamond (PCD) material or polycrystalline cubic boron nitride (PCBN) material, which are the hardest cutting tool materials, is cut out (blanking), in the form of tips, from PCD or PCBN stock, and a brazing material is fused in a high-frequency induction heater, to braze thereby the blanked tip, in the form of a cutting blade, to a tool body.
Cutting (blanking) of the tips out of PCD stock (PCD disc) can be accomplished by wire electric discharge machining, as illustrated in FIG. 23A. The tip shape that is cut out of the PCD stock may be, for instance, rectangular, as illustrated in FIG. 23A, or triangular, as illustrated in FIG. 23B. This applies not only to PCD stock, but to PCBN stock as well. The PCD tip that is cut out as illustrated in FIG. 23A and FIG. 23B is brazed to a tool body through fusion of a brazing material using a high-frequency induction heater, as illustrated in FIG. 24.
In high-precision machining of a cutting edge of a rotary-type cutting tool to which there is mounted, in the form of a cutting edge, a polycrystalline diamond (PCD) material or polycrystalline cubic boron nitride (PCBN) material, which are the hardest cutting tool materials, the materials themselves are extremely hard, and hence the machining operation is difficult. In grinding machining illustrated in FIGS. 25a-b where a diamond grindstone is used, grinding must be performed for a prolonged time. Moreover, grindstone wear is significant, and machining efficiency poor, in the case of fine shapes or complex and intricate machining shapes.
As a result, known machining methods for manufacturing PCD tools include electric discharge machining methods in which a PCD material is machined through burning by exploiting phenomena. In this context, discharge polishing machines are also known in which a disc of alloy of copper and tungsten is made into a disc electrode according to a desired PCD tool outline shape, and electric discharge machining is performed thereupon through application of pulsed voltage between the disc electrode and the PCD tool.
However, an expensive disc electrode must be prepared every time, in accordance with the PCD tool shape, and the entire shape must be burned through discharge. The machining time of deep-groove shapes or the like is thus extremely long. Also, a plurality of disc electrodes are required, on account of disc electrode consumption, which is a further factor that drives up the machining unit cost of the tool.
Therefore, PCD tool machining by wire electric discharge machining, in which cutting proceeds along a machining path alone, even for a complex shape, as illustrated in FIG. 26A and FIG. 26B, is a very convenient machining method, since electrode preparation can be dispensed with, and machining time can be shortened.
Upon machining of a cutting blade by wire electric discharge machining, as compared with grinding machining and discharge polishing machining, a relief angle is imparted to the outer peripheral shape of the tool by a wire electrode that is stretched, along a straight line, with respect to the rake face of the PCD tool that is being machined. To that end, flanks must be machined by taper machining with a tilted wire electrode. Therefore, this mandates that the rake face position of the PCD tool be measured accurately, and that a wire electrode be positioned accurately with respect to that position. The operation involved, however, is not easy. If the position of the rake face is not measured accurately, a machining error ensues as a matter of course. This machining error becomes manifest mainly in the form of a rotation radius precision error of the cutting tool, upon rotation of the tool.
How to secure the shape precision of the cutting blade of the tool is a key requirement in terms of obtaining a tool of higher precision and longer life in machining of a cutting tool using a wire electric discharge machine.
Japanese Patent Application Publication No. 11-267925 discloses a technology that involves measuring the rake face of a tool using a touch sensor or the like, and performing machining then on the basis of the measurement result. In the technology disclosed in Japanese Patent Application Publication No. 11-267925, two sites per blade are measured, in the tool radial direction, in order to position the rake face horizontally; then, while keeping the rake face horizontally from that position, the misalignment between the tool center height and the rake face height at that time is calculated, and on the basis of the calculation result, the wire position during machining is corrected through displacement in the radial direction in such a manner that the outermost peripheral diameter of the tip of the blade takes on a desired value.
Japanese Patent Application Publication No. 2003-117733 discloses a machining method in which the rotation angle upon travel across the distance from a machining start point up to a machining ending point is calculated, by arithmetic processing, on the basis of the difference between the rake face height at a position adjusted using an angle indexing device in such a manner that the top face of one end section, being a machining start position of a PCD tip rake face that is mounted to a cutting tool, becomes horizontal according to a dial gauge, and the height of the rake face at the other end section as obtained thereafter by moving the dial gauge to the machining end point at the other end section.
Japanese Patent Application Publication No. 9-267219 discloses a method in which a cutting blade edge section positioned at the outer periphery, in the rotation direction, of the end section of a tool leading end is observed using a microscope mounted to the tool leading end side, to check thereby the positioning, in the rotation direction, of the cutting blade that is to be machined, even though the machining shape of the cutting tool, called a formed shape, is different from the above-described linear shape.
How to secure the shape precision of the cutting blade of the tool is a key requirement in terms of obtaining a tool of higher precision and longer life in machining of a cutting tool using a wire electric discharge machine.
No major problems arise in the case of a machining shape in which the outer peripheral diameter of the shape is constant and the machining shape is a linear shape that is parallel to the tool center axis, for instance in a reamer tool, as in the technology disclosed in the abovementioned Japanese Patent Application Publication No. 11-267925. In the case of cutting tools having complex outer peripheral shapes, in particular circular arc shapes, however, instances arise in which mere correction in the radial direction by circular arc quadrants is inadequate. These circular arc quadrants denote sites at which the rotation radius changes gradually from the rotation center axis (FIG. 4A). In the case of elongated blade shapes, an elongated PCD tip becomes often brazed curvedly, on account of heat during brazing of the PCD tip, so that high tool radius precision cannot be achieved unless this curved portion is measured (FIG. 27).
In the technology disclosed in the above-described Japanese Patent Application Publication No. 2003-117733, the rotation angle is worked out on the basis of the radius and the difference between height and the distance from a machining start point to a machining ending point. Therefore, this method can be used only for a linear shape for which the tool radius does not change. Needless to say, the degree of curving of a long PCD tip that is brazed cannot be worked out accurately even in the case of a linear shape, as in the technology disclosed in Japanese Patent Application Publication No. 11-267925 (FIG. 28), as a result, the associated error translates into a tool radius error, thereby making it difficult to produce a tool with high precision. However, no correction means are provided herein, let alone for a machining shape of a cutting blade having a circular arc shape.
The above-described technology disclosed in Japanese Patent Application Publication No. 9-267219 cannot be used in a cutting tool of which the rake face curves halfway, or of which the rake face is in a raised-center or lowered-center state such that the rake face runs through the tool center line but is not parallel thereto (see Japanese Patent Application Publication No. 11-267925), or in a cutting tool of which the rake face is tilted, as in Japanese Patent Application Publication No. 2003-117733 described above. It is also evident that a rotating tool having a spiral cutting blade, such as the one illustrated in FIG. 28, is difficult to machine, with high precision, in the above machining methods.